As discussed in the previous Section, the predictions for the
\(cg \rightarrow cg\)  mechanism are calculated assuming that the partons
in the final state are massless, which implies introduction of a
cutoff \(p_{t0}\)  to regularize the singularity in the partonic cross
section (see [1]}). In order to constrain this parameter,
we will initially consider the LEBC - MPC data [2]}
for the \(D\)  meson production in \(pp\)  collisions at \(\sqrt{s} = 39\)  GeV.
In Fig. REF  we present our predictions for
the \(x_F\)  and \(p_T\)  distributions of the charm meson, obtained using
the CT14nnloIC parametrization for \(P_{ic} = 1\%\)  in the calculation of
the \(cg \rightarrow cg\)  mechanism. The results for the \(x_F\) 
distribution indicate that the inclusion of the \(cg^* \rightarrow cg\) 
mechanism is needed in order to describe the data. Moreover,
the \(p_T\)  distribution is also well described.
Both results point out that a value of \(p_{t0} = 2.0\)  GeV is a good
choice for the cutoff, which can be considered conservative, since
smaller values imply a larger amount for the contribution of
the \(cg \rightarrow cg\)  mechanism. Such choice is also justified
by the recent analysis performed in Ref. [3]},
where a comprehensive study of the impact of an intrinsic charm
component on the D meson production in \(pHe\)  fixed - target collisions
at the LHCb was performed. The results presented in
Ref. [3]} indicate that the LHCb data can be
well described assuming \(p_{t0} = 2.0\)  GeV for a probability of
1% of finding a charm quark-antiquark pair in the proton wave function.
